Qrp Gaijin’s post goes into some detail about his circuit and shows the schematic as well. He also shows the results with an active loop antenna, RTL-SDR and the upconverter in some videos. In the email to us he also notes that his upconverter is still a work in progress as the LO is quite noisy and he suspects that it may be too weak to drive the 1N4148 based diode ring mixer. There is also no filtering on the circuit yet, so there is some broadcast FM breakthrough.
Another project he worked on was attempting the direct sampling mod on a standard RTL-SDR. However, Qrp Gaijin’s method is slightly different to most attempts as instead of soldering the wires into the Q-branch holes he simply uses hot glue to hold them mechanically in place. This may be an idea to consider for those who want to attempt the mod on a standard RTL-SDR, but don’t have any soldering tools or experience.
The BA5SBA direct sampling kit is a kitset PCB that combines with a standard (included) RTL-SDR dongle in order to enable the direct sampling mod. The direct sampling mod is a hardware modification that can be applied to any RTL-SDR dongle in order to enable HF reception capabilities. The BA5SBA kit improves upon some of the problems with the direct sampling mod by adding additional features such as a low pass filter to block broadcast FM interference, a matching transformer to better match the RTL2832U’s input impedance, extra power supply filtering, SMA connectors for HF and VHF/UHF (UV), an aluminium case and a bias tee.
The BA5SBA direct sampling RTL-SDR can be bought as a kit that requires hand assembly for about $30 USD or as a fully assembled product for about $50 USD. It is usually listed on Amazon and eBay as a “100KHz-1.7GHz full band UV HF RTL-SDR USB Tuner Receiver/ R820T+8232 Ham Radio”
Recently, RTL-SDR.com reader Simon (MW0SGD) bought one of these kits and discovered that the English instructions were very rare and hard to come by. We’ve decided to post these English instructions here for any future buyers who may search for them as this post should show up on Google. Simon also notes that “most of the instructions on the internet wind the inductors on a 5mm former. This says to use a 3mm one, which I did and it works ok.”
The direct sampling mod allows you to listen to the HF frequencies between 0 – 14 MHz on an RTL-SDR by simply connecting an antenna directly to the ADC pins on the RTL2832U chip. Until recently the impedance of these pins was unknown, but most people assumed that it was about 300 Ohms.
This inspired Martin to do a proper measurement of the input impedance. Martin’s measurements found that the differential input impedance of the RTL2832U is approximately 3,330 Ohms when the input is enabled, and this would require a 66:1 transformer. However, Martin writes that a wideband transformer like this probably does not exist, but that the T16-6T-KK81 with terminating resistors added is probably a good choice.
Recently RTL-SDR.com reader Dr. Phil wrote in to let us know about some PDF notes that he has created about the RTL-SDR dongle. There is some good information in his documents and the notes mainly focus on using the RTL-SDR with the direct sampling mod to receive HF.
His other documents also explain concepts such as imaging, interference and gain, how to reduce interference, input impedance of the Q sampling pads, intermediate frequency, and sample rate. In addition he has also uploaded some documents where he has calculated for various AM, FM and SW stations at what frequencies images will show up. His final document also discusses the Mirics SDR chipsets which are used in the SDRPlay.
The lowest frequency that a standard RTL-SDR dongle can receive is about 24 MHz. However, by applying a hardware hack called the direct sampling mod, it is possible to use the RTL-SDR to listen to the HF frequencies.
Along with the diplexer Martin added an impedance transformer, added additional coupling capacitors to the power rails and removed the IR LED components to make space for the transformer. Martin writes that the final modded RTL-SDR allows for tuning between 15 kHz to 1.8 GHz.
Over on the Reddit RTL-SDR discussion board user pksato has posted an image of his direct sampling modified RTL-SDR dongle. His mod includes a simple common base amplifier with 9dB gain, an impedance transformer with calculated output impedance of 50 Ohms and a 5V power filter.
The direct sampling mod allows you to modify your RTL-SDR to receive HF frequencies, without the need for an upconverter. It requires a hardware mod that can be as simple as soldering a wire to one of the RTL2832U pins, but for improved results you will need filters, impedance transformers and amplifiers. There is more information about direct sampling here.
We also note that amateur radio hobbyist KN0CK (aka Marty Wittrock) sells direct sampling modified dongles that are modified in a similar way to pskato’s mod (circuit placed on top of the RTL-SDR PCB). His products can be found at http://www.kn0ck.com/HF_SDR/.
YouTube user opilarczyk has posted a video comparing HF performance of the rtl-sdr with the direct sampling mod at 7MHz with WebSDR, an online SDR streaming site. The comparison shows how the direct sampling mod is extremely susceptible to broadcast FM interference.
The RTL-SDR software defined radio can be told to run in a mode called “direct sampling mode”, which with a small hardware mod allows the dongle to tune to the HF frequencies where ham radio and many other interesting signals are found. This means that no upconverter circuit is required.
However, the difficulty with direct sampling is that a hardware modification to the dongle is required. Also, the performance can not be expected to be as good as an upconverter without the addition of extra filtering circuits.